1. Field of the Invention
The present invention relates to an imaging lens. In particular, the present invention relates to an imaging system of a two-lens structure that is capable of reducing size and weight, enhancing optical performance, and improving productivity. The imaging lens is used for an image-taking device that forms an image of an object, such as scenery or a person, on an image-taking surface of a solid image pickup device such as a CCD, CMOS, etc. The solid image pickup device is mounted on a portable computer, a television phone, a portable phone, a digital camera, and the like.
2. Description of the Related Art
Recently, there has been an increasing demand for cameras that utilize a solid image pickup device, such as a CCD, CMOS, or the like, which is mounted on a portable phone, a portable computer, and a television phone, for example. It is demanded that a camera such as this is small and light because the camera is required to be mounted on a limited installation space.
Therefore, it is also necessary for the imaging lens used for such cameras to be similarly small and light. Conventionally, a single-lens structure lens system using a single lens is used as such imaging lens.
Such a single-lens structure lens system can sufficiently handle being applied to a solid image pickup device called CIF that has a resolution of about 110 thousand pixels. However, in recent years, the use of a solid image pickup device called VGA that has a high resolution of about 300 thousand pixels is being examined. There is a problem in that the conventional single-lens structure lens system cannot sufficiently exert the resolution performance of such high-resolution solid image pickup devices.
Therefore, conventionally, various two-lens structure lens systems and three-lens structure lens systems having an optical performance that is superior to that of the single-lens structure lens system have been proposed.
In this case, in the three-lens structure lens system, each aberration leading to the deterioration of the optical performance can be effectively corrected, thereby allowing an extremely high optical performance to be obtained. However, since the three-lens structure lens system has a large number of parts, it is difficult to reduce the size and weight. Since each component part requires high accuracy, the manufacturing costs increase, as well.
On the other hand, although the two-lens structure lens system cannot achieve an optical performance that is as high as that of the three-lens structure lens system, a higher optical performance than that of the single-lens structure lens system can be acquired. It can be said that the two-lens structure lens system is suitable for a small, high-resolution solid image pickup device.
As such a two-lens structure lens system, conventionally, numerous lens systems called a retro-focus-type in which a negative lens and a positive lens are combined have been proposed. It is possible to reduce costs of such retro-focus-type lens systems through a reduction of the number of parts. However, from this configuration, it is practically impossible to reduce the size and weight to about the size and weight of the single-lens structure lens system since the back focus distance increases.
As another two-lens structure lens system, there is a lens system called a telephoto-type in which a positive lens and a negative lens are combined. However, such telephoto-type lens systems had been developed for silver-salt photography, and therefore, the back focus distance of the telephoto-type lens system is too short. The telephoto-type lens system also has problems regarding telecentricity. It is difficult to apply the telephoto-type lens system as is to the imaging lens for solid image pickup devices.
Furthermore, conventionally, in the two-lens structure lens system or the three-lens structure lens system, a configuration in which a diaphragm is disposed between two lenses that are mutually adjacent in the optical axis direction is mainstream (for example, refer to Patent Literatures 1 and 2).    [Patent Literature 1] Japanese Patent Unexamined Publication 2004-163850    [Patent Literature 2] Japanese Patent Unexamined Publication 2004-170460
There is an increasing demand for further improvement of the optical performance, in addition to the reduction in size and weight, of these types of imaging lens. However, in a configuration in which the diaphragm is disposed between the two lenses, as in the imaging lens described in Patent Literatures 1 and 2, it is difficult to achieve both the reduction in size and weight and further improvement in the optical performance. Furthermore, it is difficult to accommodate the sensor characteristics (incident angle to the sensor).